Formula where the LHS indicates the name of the mortality rate
column, and the RHS is the column where age is located. This function
assumes that the population data frame has already been stratified by sex,
or any other categorical variable of interest.
data
Data frame of population mortality stratified by sex and age. The
following columns must be present: sex, age, and rate.
max_age
Maximum age to calculate mortality for.
Value
An estimate of the survival rate by age in days, with max_age
* DAYS_IN_YEAR values.